1. Field of the Invention
The invention relates to a method for the production of a seam in a workpiece, the seam comprising a starting point and an end point; the seam being produced by means of a sewing machine, which comprises a hook and a needle having an eye; the seam further comprising a needle thread supplied to the eye and a looper thread; and a front end of the needle thread extending out of the eye of the needle prior to the start of production of the seam; the method comprising the following steps: production of the seam; stopping the sewing machine with the needle outside the workpiece, the needle thread extending from the end point of the seam to the eye of the needle, and the looper thread extending from the end point of the seam toward the hook; cutting the needle thread by means of a first cutter and cutting the looper thread by means of a second cutter, short remaining threads remaining on the workpiece. The invention futher relates to an apparatus for putting the method into practice, the apparatus comprising a sewing machine which has a hook and a needle, driven to reciprocate up and down and having an eye, for the production of a seam in a workpiece, the seam comprising a needle thread supplied to the needle and a looper thread, the seam taking a course which has a starting point and an end point, and, prior to the start of production of the seam, a front end of the needle thread being provided that extends out of the eye of the needle; a driving motor for the sewing machine for driving and stopping the sewing machine with the needle out of the workpiece; a first cutter for the needle thread and a second cutter for the looper thread of the seam, the workpiece, after the cutting of the needle thread and the looper thread, having short remaining threads, and a control unit connected with the driving motor and the cutters.
2. Background Art
A sewing machine is known from a leaflet, Durkopp Adler AG 504, imprint D/GB/F 11.90, putting into practice the method of the generic type of producing a two-thread lock stitch seam for a group of stitches, for instance a lock stich seam on safety belts. Such a stitch group consists of a number of stitches lined up to form a seam taking a given, closed course. Stitch groups of this type are required to provide for a certain solidity of seam and to offer the possibility that any thread tail pieces produced are cut off as short as possible where extending out of the workpiece. To this end, the sewing machine is provided with thread cutters working close to the workpiece surfaces and being embodied for instance as thread burn-cutters.
For the requirements of solidity of seam to be fulfilled, very tight stitches must be produced by the needle thread being pretensioned correspondingly. For this to be carried out controllably, attention must be drawn to the fact that the friction occurring during stitching is not too high between the thread and the workpiece, which is again influenced by the thickness of the needle. This is opposed by the fact that too low a friction between the thread and the workpiece will prevent regular stitching at the beginning of the seam.
So as to obtain sufficient solidity of seam, the beginning and the end of the seam are usually secured against becoming undone. A way of securing a seam consists in that stitches are produced at the beginning and end of the seam, having a reduced stitch length or no stitch length at all. Other ways of securing a seam consist in that the stitches produced at the beginning and end of a seam overlap each other. When synthetic threads are used, for instance in safety belts, additional securing of a seam results from the fact that the tail pieces cut off by a thread burn-cutter are thickened.
Owing to the conditions described, in these stitch groups, an accumulation of threads will inevitably result at the joint of the seam, i.e. where the starting point and the end point of the seam are close to each other. This will affect the quality and appearance of the stitch group, which becomes especially unpleasant when thread of some thickness, for instance of a diameter of 0.8 mm, are used.
U.S. Pat. No. 5,345,887 teaches a method of producing a seam in which, at the beginning of seam production, a thread clamp acts periodically on the thread supplied to the needle from a thread supply, withdrawing the thread by a certain length so that any excess of needle thread is avoided to accumulate on the lower side of the workpiece, this aiming at ensuring an impeccable appearance of the seam on the lower side of the workpiece. In this case, a needle-thread end piece is produced at the starting point of the seam, which leads through the workpiece, there however being insufficiently secured, so that it is necessary, for a certain solidity of seam to be ensured, to sew again along the starting point of the seam.